Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to a person skilled in the art and can be based on various effects. Such devices are, for example, cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells ("super-twisted nematic"), SBE cells ("super-birefringence effect") and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical, photochemical and thermal stability and a good stability towards electric fields. Further, they should have a suitable mesophase over a broadest possible range (for example, a nematic or a cholesteric phase for the cells mentioned above), but nevertheless should have a sufficiently low viscosity and in the cells should permit short response times, low threshold potentials and a high contrast. Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the field of application and type of cell. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as low as possible. For DAP cells components having a negative dielectric anisotropy are necessary.
In order to fulfil these to some extent contradictory requirements mixtures with up to about 15 components must generally be produced. It is therefore important that the components have a good miscibility with one another and have a sufficient solubility.
In order to achieve a suffiently broad mesophase range there must be added to the mixtures mainly clearing point-increasing components which, however, can disadvantageously influence the viscosity and the electro-optical properties. Further, materials having a low optical anisotropy, which are of interest e.g. for actively addressed liquid crystal indicators, frequently give smectic tendencies and lead for the most part to an increase in the threshold potential and/or the response times. Furthermore, non-polar materials with high optical anisotropy often have only smectic mesophases or even no liquid crystalline properties.